Field of the Invention
The present invention relates to a liquid electrolyte for electric double-layer capacitors and to an electric double-layer capacitor.
Description of the Related Art
An electric double-layer capacitor is a capacitor in which an electric double layer that appears at the interface between a conductive electrode and a liquid electrolyte is constituted as a dielectric for storing electric charge thereon.
An electric double-layer capacitor unlike a lithium rechargeable battery entails no chemical reaction in storage of charge. This allows the electric double-layer capacitor to charge and discharge rapidly and its cycle life to be long, making it suitable for use in a power-assisting and an energy-regenerating device which requires an instantaneously increased electric current.
An electric double-layer capacitor in the prior art has problems, however.
The first problem is that it has a relatively low electrostatic capacitance. In order to increase the electrostatic capacitance, improvements have been made by contriving to achieve an increased specific surface area of an electrode material to acquire an electrostatic capacitance as of a lithium ion rechargeable battery.
It is, however, desirable to implement an electric double-layer capacitor which possesses a further increased electrostatic capacitance.
The second problem is that a conventional electric double-layer capacitor has a low dielectric strength or withstanding voltage such that the voltage to which it can be charged may at the maximum be up to 3 volts or so. While enhancing the storage energy of a capacitor is thought to include raising the voltage that can be applied, applying a voltage as high as, say, 3 volts, to an electric double-layer capacitor using a liquid electrolyte having an electrolyte dissolved in an organic solvent comes to deteriorate its durability and the like.
An ionic liquid that is a salt which exhibits a liquid phase in the vicinity of room temperature and which is nonvolatile, noncombustible, thermally stable, ion-conductive and resistant to electrolysis, is better in resistance to oxidation than a usual organic solvent. Thus, using an ionic liquid as a liquid electrolyte than using a liquid electrolyte having an electrolyte dissolved in a usual organic solvent can be expected to improve the durability of a capacitor under the condition that a high voltage of around 3 volts is applied thereto (See, for example, Patent Document 1 infra).
However, for an electric double-layer capacitor using an ionic liquid as its liquid electrolyte it is desired to have a larger electrostatic capacitance as well. Also, its durability under higher applied voltage conditions of exceeding 3 volts is insufficient, requiring further improvements.
As a system improving a condition of voltage application, it should in passing be noted that a lithium ion capacitor using lithium ions has been adopted. However, the problems arises that applying a voltages of 4 volts or more causes metal lithium to be deposited.
As regards a fullerene, processes of its synthesis and isolation have illustratively been disclosed in Patent Documents 2 and 3 infra, respectively.